The present invention relates to transparent, monolithic porous glass, particularly a process for making large sizes of transparent porous glass monoliths from gel, and more particularly to such a process which depends on using temperature to control the partial pressure of the gel solvent in a closed vessel, resulting in controlled shrinking during drying.
Porous glass has been developed for applications ranging from lightweight optics to nuclear particle detectors and sorption media. Highly porous glass (i.e., porosity >85%) called aerogels, typically require special techniques to successfully dry large uncracked pieces. Intermediate porosities (i.e., porosity >40% but <85%) called xerogels, usually require long drying times (e.g., several weeks for pieces > a few square centimeters). Also, such xerogels are not very transparent in a particular range of porosities (40% to 85%) due to scattering from aggregates within the gel. Thus, new methods are needed to rapidly process porous glass and also to fabricate transparent porous glass having specific refractive indexes.
The present invention provides a method for rapidly making transparent porous glass monoliths from gels. The glass can be produced much faster and in much larger sizes than by present technology for making porous glass. The method of this invention depends on using temperature to control the partial pressure of the gel solvent in a closed vessel, resulting in controlled shrinkage during drying. The final density of the produced transparent porous glass monoliths is determined by the recipe for the gel and the time and temperature. The method of this invention involves preparing gels which shrink during drying due to strong capillary forces at the liquid-solid-vapor interface.